1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting liquid to a printing medium.
2. Description of the Related Art
In recent years, an ink jet printing apparatus has been increasingly in widespread use in which ink is ejected from a print head to cause ink to adhere to the printing medium to thereby form an image for printing. Generally, an ink ejection method in the ink jet printing apparatus includes, for example, a bubble jet method for heating ink to boil the ink to use the foaming force thereof and a piezo method using the displacement of a piezoelectric body when an electric field is applied to the piezoelectric body.
The ink jet printing apparatus is advantageous in that a print head can have a compact size and a high-definition image can be printed at a high speed for example. The ink jet printing apparatus is also advantageous in that multiple color inks can be used to print a color image easily. Recently, ink ejected from the ink jet printing apparatus has an increased ejection frequency. Furthermore, the printing image by the ink jet printing apparatus also has an increased resolution. Thus, there has been a tendency where the electric power inputted to the print head increases. Furthermore, the print head in the ink jet printing apparatus has a reduced size in order to reduce the manufacture cost of the print head. In accordance with this, there has been a tendency where the heat value per a unit area of the print head further increases.
Generally, a change in the ink characteristic is caused in response to a change in the ink temperature. Thus, there may be a case where the characteristic of the ejected ink changes in response to the change in the print head temperature during ink ejection. Regarding this, Japanese Patent Laid-Open Publication No. 2007-168112 discloses an ink jet print head attached with a heat dissipation member having a thermal conductivity in order to dissipate heat to the outside of the print head. As described above, there is an approach to suppress an excessive temperature increase in the print head during printing to thereby stabilize the ink ejection by the print head.
A print head disclosed in Japanese Patent Laid-Open Publication No. 2006-159893 is widely known. The print head having this form is structured so that a plurality of ejection port arrays each of which consists of a plurality of ejection ports are arranged and an ink supply ports extending along the ejection port arrays are formed among the ejection port arrays. Ink is fed from the ink supply ports to flow paths and pressure chambers communicating with the respective ejection ports constituting ejection port arrays at both sides thereof. Then, heaters provided in the pressure chambers are caused to generate heat to thereby eject ink. This configuration is advantageous in that the ejection ports can be arranged at a relatively high density.
The following section will consider a print head having a configuration as disclosed in Japanese Patent Laid-Open Publication No. 2006-159893 in which ink supply ports extend among ejection port arrays along the ejection port arrays and the plurality of ejection port arrays and ink supply ports extend to be parallel to one another. When such a print head is used, heat from those printing elements provided at the centers of the respective ejection port arrays is suppressed from dissipated in the direction along which the ejection port arrays extend. The printing elements at the centers of the respective ejection port arrays have neighboring printing elements in the vicinity of the respective printing elements with regard to the direction along which the ejection port arrays extend. Thus, since the neighboring printing elements also emit heat to have a high temperature, the temperature gradient is suppressed from occurring in the direction along which the ejection port arrays extend.
Furthermore, in the case of the print head having this configuration, the ink supply ports are formed among the ejection port arrays and the plurality of ejection port arrays are formed to be parallel to one another. Thus, there may be a case where an ejection port array both sides of which are sandwiched by ink supply ports is formed. Generally, ink has a thermal conductivity much lower than that of a silicon substrate or an alumina chip plate forming a print head. Heat generated from the printing element is easily transmitted to the silicon substrate and the alumina chip plate. However, heat generated from the printing element is difficult to be transmitted to ink stored in an ink supply port. Due to this reason, in the print head in which ejection port arrays are arranged in the manner as described above, the heat generated from the printing element is difficult to be transmitted to the ink in the ink supply ports formed so as to sandwich the ejection port array. Thus, there may be a case where the ink supply ports may suppress heat from being dissipated in the direction orthogonal to the direction along which the ejection port arrays extend. As described above, the heat generated from the printing elements at the centers of the respective ejection port arrays is difficult to be dissipated. Thus, the centers at the ejection port arrays tend to have an increased temperature.
On the other hand, at the ejection ports formed at ends of the respective ejection port arrays, heat is dissipated to the outside of the ejection port arrays. As described above, a relatively large amount of heat is dissipated from those ejection ports formed at positions close to ends at the outer sides of the ejection port arrays. Thus, the ejection ports formed at positions close to ends at the outer sides of the ejection port arrays have a lower thermal resistance than the ejection ports at the centers of the ejection port arrays. Thus, a temperature increase is suppressed at the positions close to ends at the outer sides of the ejection port arrays. With regard to the direction along which the ejection port arrays extend, the centers of the ejection port arrays have a high thermal resistance and tend to have a temperature increase relatively easily. The outer sides of the ejection port arrays on the other hand have promoted heat dissipation to thereby relatively suppress a temperature increase.
Furthermore, in the print head having the configuration as described above, when the ejection port arrays formed in the print head are compared to one another, the heat dissipation amount from those ejection port arrays formed at the outer side of the substrate among the plurality of ejection port arrays is higher than that from those ejection port arrays formed at the inner side of the substrate. Thus, the ejection port arrays formed at the outer side of the substrate tend to have a suppressed temperature increase. The reason is that, according to the print head having the configuration as described above, an ejection port array formed at the inner side of the substrate is formed so as to be sandwiched between two ink supply ports. As described above, a silicon substrate or an alumina chip plate forming a print head has a thermal conductivity much lower than that of ink. This may cause a case where an ink supply port may hinder heat dissipation in the ejection port arrays formed at the inner side of the substrate. Thus, heat generated from the printing element in the ejection port arrays formed at the inner side of the substrate is dissipated in a relatively small amount. In the ejection port arrays formed at the outer side of the substrate on the other hand, heat is easily dissipated to the outer side of the substrate. Specifically, heat generated from the printing element of the ejection port arrays formed at the outer side of the substrate tends to be dissipated from the printing element in a direction orthogonal to the direction along which the ejection port arrays extend. As described above, among the plurality of ejection port arrays, a relatively high amount of heat is dissipated from the ejection port arrays formed at the outer side of the substrate and thus a temperature increase is suppressed therein. On the other hand, a relatively-small amount of heat is dissipated from the ejection port arrays formed at the inner side of the substrate and thus a temperature increase is easily caused therein.
When the temperature distribution is uneven depending on each region of the print head as described above, ejected ink may have characteristics that are different depending on the respective regions where the ejection ports are formed. Thus, there is a possibility where the respective regions of the print head have an uneven distribution of the ejection performances from the ejection ports, thereby causing an image obtained by printing to have a deteriorated quality.